Field of the Invention
The invention relates to a method for determining a pulse duration T90 of a 90° pulse in a nuclear magnetic measuring method with a circuit arrangement. Thereby, the circuit arrangement has a signal generator for generating the 90° pulse, a coil for transmitting the 90° pulse to a medium, a coupling circuit and an angular resonance frequency ω0. The signal generator has a known generator resistance RS. The coil has a coil impedance ZL with a coil resistance RL and a coil reactance XL according to ZL=RL+jXL. The coupling circuit has a matching capacitor having an adjustable matching capacitance CM and a tuning capacitor having an adjustable tuning capacitance CT. The medium is magnetized by a magnetic field for the nuclear magnetic measuring method and has a Larmor precession having an angular Larmor frequency ωP.
Description of Related Art
The invention further relates to a nuclear magnetic flowmeter for a nuclear magnetic measuring method having a circuit arrangement and a control unit. Thereby, the circuit arrangement has a signal generator for generating a 90° pulse, a coil for transmitting the 90° pulse to a medium, a coupling circuit and an angular resonance frequency ω0. The signal generator has a known generator resistance RS. The coil has a coil impedance ZL with a coil resistance RL and a coil reactance XL according to ZL=RL+jXL. The coupling circuit has a matching capacitor having an adjustable matching capacitance CM and a tuning capacitor having an adjustable tuning capacitance CT. The medium is magnetized by a magnetic field for the nuclear magnetic measuring method and has a Larmor precession having an angular Larmor frequency ωP.
The circuit arrangement has an angular resonance frequency ω0, since it is designed as a resonant circuit.
A Nuclear magnetic measuring method requires that a medium, for which the nuclear magnetic measurements of the method are to be carried out, contains atomic nuclei that have a nuclear spin and thereby also a magnetic moment. The nuclear spin of an atomic nucleus is understood as an angular momentum describable by a vector and, accordingly, the magnetic moment of the atomic nucleus is described by a vector that is aligned parallel to the vector of the angular momentum. In the presence of a magnetic field, the vector of the magnetic moment of the atomic nucleus tends to be aligned parallel to the vector of the magnetic field at the location of the atomic nucleus. Thereby, the vector of the magnetic moment precesses around the vector of the magnetic field at the location of the atomic nucleus. The precession is known as Larmor precession. The angular frequency of the Larmor precession is called angular Larmor frequency ωP and is the product of the gyromagnetic ratio and the absolute value of the magnetic flux density at the location of the atomic nucleus.
Nuclear magnetic measuring methods are based on the magnetization, by means of a magnetic field, of a plurality of atomic nuclei each having a magnetic moment in a volume of a medium. In the absence of a magnetic field, the individual alignment of the vectors of the magnetic moments is statistically uniformly distributed, which is why the medium is not magnetized in the volume. The presence of a magnetic field disturbs the statistically uniform distribution of the individual alignment of the vectors of the magnetic moment, whereby a magnetization parallel to the magnetic field is formed in the medium in the volume. Macroscopic magnetization is understood in general under magnetization. The temporal course of the process of alignment of the individual vectors of the magnetic moments in a magnetic field is characterized by the spin-lattice relaxation time constant and has an exponentially decreasing course. The values of the spin-lattice relaxation time constants are characteristic for different substances, wherein substances can also be called phases.
Nuclear magnetic measuring methods determine, for example, the flow of a medium through a measuring tube of a nuclear magnetic flowmeter or, in a medium having several phases, i.e. at least two phases, the portions of the individual phases in the medium. A combination of nuclear magnetic measuring methods is also possible. For example, a nuclear magnetic measuring method determines the portions of the individual phases of the medium as well as the flow of the individual phases through the measuring tube for a medium having several phases. Both the mass flow and the volume flow of a medium are called flow.
In a medium having several phases, determining the portion of the individual phases in the medium does not only require that each of the phases has atomic nuclei with magnetic moments, so that the phases can be magnetized in a magnetic field, but also that the individual phases of the medium have different spin-lattice relaxation time constants, so that the individual phases can be differentiated from one another. The medium extracted from oil sources, for example, consists essentially of the liquid phases crude oil and saltwater and the gaseous phase natural gas. Thereby, all phases contain hydrogen atom nuclei. Since hydrogen atom nuclei have the greatest gyromagnetic ratio of all atomic nuclei and the individual phases also have different spin-lattice relaxation time constants, nuclear magnetic flowmeters are suitable, in particular, for the measurement of flow of the multi-phase medium extracted from oil sources. Phases having hydrogen atomic nuclei are, in particular, suitable for nuclear magnetic measuring methods, however, phases having atomic nuclei with a smaller gyromagnetic ratio than that of hydrogen atomic nuclei, such as sodium atomic nuclei, are also suitable for nuclear magnetic measuring methods.
Nuclear magnetic measuring methods generally also include nuclear magnetic measurements of the magnetization of a medium in a volume after a certain exposure duration to a magnetic field. Such a nuclear magnetic measurement requires the previous rotation of an angle of 90° of the vectors of the magnetic moments of the atomic nuclei of the magnetized medium in the volume in relation to the vector of the magnetic field. The vectors of the magnetic moments rotated at an angle of 90° cause a measuring signal in a sensor, which represents the magnetization of the medium in the volume. Such a sensor is, for example, a sensor coil, in which the magnetic moments precessing with the angular Larmor frequency ωP induce a voltage as measuring signal. The strength of the measuring signal is at a maximum at a rotation of an angle of 90° and becomes less when the rotation of an angle of 90° does not occur.
A rotation of the vectors of the magnetic moments of the atomic nuclei of the magnetized medium in the volume in respect to the vector of the magnetic field is carried out using an electromagnetic pulse, to which the magnetized medium in the volume is exposed. Such a pulse has an angular frequency ωK that corresponds to the angular Larmor frequency ωP of the medium, whereby a torque acts on the magnetic moments of the atomic nuclei of the medium in the volume, which then causes the rotation. The angle of rotation is specified by the pulse duration of the pulse. The pulse duration is, thereby, to be differentiated from the period duration TK=2π/ωK of an individual electromagnetic oscillation of a pulse. An electromagnetic pulse that causes a rotation of an angle of 90° is called a 90° pulse and the pulse duration is called pulse duration T90. The pulse duration T90 is, thereby, in particular dependent on the medium. A 90° pulse has exactly one single pulse duration T90.
It is known from the prior art to determine the pulse duration T90 required for a given medium in that a measuring series of nuclear magnetic measurements of the magnetization at different pulse durations is carried out in a volume of the medium with a constant magnetization. The pulse duration that causes a rotation of the magnetic moments of an angle that comes closest to an angle of 90° is the pulse duration, at which the strongest measuring signal of the measuring series is measured. This pulse duration T90 is assigned to the pulse duration to be set for a rotation of an angle of 90°. The prior art has, in particular, the disadvantage of requiring a large amount of time for carrying out the measuring series.